Nitrile rubbers, also referred to in abbreviated form as “NBRs”, are rubbers which are copolymers or terpolymers with at least one α,β-unsaturated nitrile, at least one conjugated diene and optionally one or more other copolymerizable monomers. Hydrogenated nitrile rubber, also referred to in abbreviated form as “HNBR”, is produced by hydrogenating nitrile rubber. Correspondingly, in HNBR, the C═C double bonds of the copolymerized diene units are wholly or partly hydrogenated. The degree of hydrogenation of the copolymerized diene units is situated typically in a range from 50% to 100%.
Processes for producing such nitrile rubbers are known—see, for example, Ullmann's Encyclopedia of Industrial Chemistry, VCH Verlagsgesellschaft, Weinheirn, 1993, pp. 255-261. The polymerization to the nitrile rubbers is typically carried out as an emulsion polymerization and can be initiated free-radically by azo initiators, persulphates, organic peroxides or redox systems. The literature reference above describes redox systems as the initiator systems used predominantly, in the form, for example, of mixtures of organic peroxides, hydroperoxides or persulphates as oxidizing agents with reducing agents such as sodium dithionite or Rongalit (sodium formaldehyde-sulphoxylate). Metal salts are used in this case optionally as a cocatalyst, preferably with addition of suitable complexing agents such as sodium ethylenediaminetetraacetate, sodium nitrilotriacetate, isophthalic acid, trisodium phosphate or tetrapotassium diphosphate. Whether and, if so, to what extent the initiator system affects the properties of the rubber was not a subject of investigation and is not considered at all.
EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 of Zeon Corporation each describe nitrile rubbers based on an unsaturated nitrile and a conjugated diene. A feature common to all the nitrile rubbers is that they possess 10-60% by weight of unsaturated nitrile and a Mooney viscosity in the range of 15-150 or, according to EP-A-0 692 496, of 15-65, and all contain at least 0.03 mol of a specific C12-C16 alkylthio group per 100 mol of monomer units. The nitrile rubbers are each produced in the presence of a correspondingly constructed C12-C16 alkylthiol as molecular weight modifier, which functions as a chain transfer agent and hence is incorporated as an end group into the polymer chains. In the patent applications described, the polymerization is initiated by unspecified organic peroxides, redox initiators, azo compounds or persulphates. Any influence the initiator system might have on the properties of the rubber is not considered.
In U.S. Pat. No. 2,451,180 of the Goodrich Company the polymerization is described for the purpose of preparing poly-1,3-butadienes and copolymers of 1,3-butadiene with other copolymerizable monomers such as, for example, acrylonitrile, using small amounts of water-soluble metal salts from group VIII of the Mendeleev Periodic Table based on iron, cobalt, nickel, palladium, osmium, platinum, etc. Salts of iron, of cobalt and of nickel are specified as being preferred salts. Typical examples of such salts that are recited are, generally, chlorides, nitrates, iodides, bromides, sulphates, sulphites, nitrites and thiocyanates of iron, cobalt, nickel and other group VIII metals, provided that they are water-soluble. With regard to metals which are able to occur in more than oxidation state it is said, generally, that they might be used both in the oxidized form and in the reduced form, for example, as iron(II) sulphate or iron(III) sulphate. It is said that when these metal salts are used, irrespective of whether other common initiators and accelerants are present or not, there is a considerable increase observed in the polymerization rate for a few hours, and, furthermore, that polymerization can be carried out at a low temperature of 20 to 30° C., leading, it is said, to polymers having improved properties, more particularly an improved elongation at break and tensile strength. In the examples, NBR is polymerized in the presence of iron ammonium sulphate, cobalt chloride or nickel sulphate, and using hydrogen peroxide. However, there are no investigations of the properties of the NBR vulcanizates.
U.S. Pat. No. 2,897,167 of Dow Chemical Company describes the emulsion polymerization of conjugated diolefins with methyl isopropenyl ketone or with a mixture of methyl isopropenyl ketone and another vinylidene monomer (e.g. acrylonitrile) to form rubbers or stable latices. In this context it is emphasized in particular that the use of the methyl isopropenyl ketone monomer is necessary in order to prevent gelling and hence the production of precoagulated polymer in the latex prepared. The polymerization is carried out in the presence of a redox system. Sodium formaldehyde-sulphoxylate is used as reducing agent, diisopropylbenzene hydroperoxide as oxidizing agent. In addition, redox-active iron is added in the form of iron(III) chloride or iron(II) sulphate. Any influence of the iron-based redox initiator system on the properties of the latex or rubber prepared is not expounded.
U.S. Pat. No. 2,968,645 of the Dow Chemical Company describes the emulsion polymerization of rubbers in the presence of α- or β-conidendrol (naphtho(2,3-c)furan-1(3H)-one). The α- or β-conidendrol is added to the polymerization in order first to increase the rate of the polymerization and second to obtain its antioxidant effect after the polymerization. Monomers used are the isomoners of butadiene and/or monoolefinic substances. For the redox system, according to the examples and the description, iron(III) chloride and iron(II) sulphate as metal salts are added to this emulsion polymerization. There is no investigation as to what effect the components of the redox system might have on the properties of the rubber.
U.S. Pat. No. 2,716,107 as well discloses the use of redox systems comprising iron(III) chloride or iron(II) sulphate for the polymerization of 1,3-butadiene and one or more vinylic monomers, such as acrylonitrile, for example.
Both NBR and HNBR have for many years occupied a firm place in the field of speciality elastomers. They possess an excellent profile of properties, in the form of excellent oil resistance, good heat resistance, outstanding resistance towards ozone and chemicals, the latter even more pronounced in the case of HNBR than of NBR. Furthermore, they possess very good mechanical properties and performance properties. For this reason they find broad use in a wide variety of application fields, and are employed, for example, in the production of seals, hoses, belts and damping elements in the automotive sector, and also for stators, borehole seals and valve seals in the oil extraction sector, and also for numerous parts in the electrical industry and in mechanical and marine engineering. A large number of different types are available commercially, and, according to area of application, feature different monomers, molecular weights and polydispersities and also mechanical and physical properties. In addition to the standard grades, speciality grades containing specific tertmonomers or having particular functionalizations are increasingly in demand in particular. There is also a demand for grades which possess good and easy processing properties, in particular a very good fluidity, and whose vulcanizates display a good profile of mechanical properties. Grades having improved fluidity have to date been producible, for example, by employing an additional metathesis process step, which allows the nitrile rubbers to be given a significantly lower Mooney viscosity and hence a lower average molecular weight. Since this is an additional process step, it is not seen as being unrestrictedly positive from an economic viewpoint, and, in terms of their profile of properties, the nitrile rubber grades obtained do not meet all of the requirements.
It was the object of the present invention, therefore, to provide a process which allows the synthesis of nitrile rubbers which possess good and easy processing properties, in particular fluidity, and whose vulcanizates exhibit a very good profile of mechanical properties.
This object is achieved by a new process for producing nitrile rubbers which uses a special redox system as initiator for the emulsion polymerization.